I. Field of the Invention
The present invention relates to a varistor and a method for manufacturing the same.
II. Description of the Prior Art
Conventionally, a varistor using a sintered body having ZnO as its major component is known. An attempt has been made to incorporate various additives in such a sintered body, thereby obtaining desired characteristics. In general, good voltage-current nonlinear characteristics and a long life performance are required for a varistor. However, a varistor which satisfies the both voltage-current characteristics and life performance has not been obtained. For example, a varistor of a sintered body having ZnO as its major component and Bi.sub.2 O.sub.3, CoO, Sb.sub.2 O.sub.3, NiO, and MnO as additives is described in Japanese Patent Disclosure No. 49-119188. However, sufficiently good voltage-current nonlinear characteristics has not been obtained.
It has also been attempted to control Bi.sub.2 O.sub.3 phase contained in such a sintered body in order to obtain desired characteristics. For example, in Japanese Patent Disclosure No. 50-131094, 10% by weight or more of the total Bi.sub.2 O.sub.3 content is transformed to the body-centered cubic system (.gamma. phase) to increase the stability against a pulse current and a DC load. However, the voltage-current nonlinear characteristics and the life performance greatly depend on the composition of the sintered body. Therefore, the overall characteristics of the varistor cannot be improved by controlling only the .gamma.-Bi.sub.2 O.sub.3 phase. In particular, satisfactory voltage-current nonlinear characteristics cannot be obtained.
In the conventional varistors, the both requirements of good voltage-current nonlinear characteristics and a long life performance cannot be simultaneously satisfied. In particular, when a varistor is used as an arrester which must absorb a high surge voltage, good voltage-current nonlinear characteristics must be provided. Furthermore, even stricter criteria are required of such characteristics in the development of ultra high-voltage (UHV) power supply.